I've been asked to test and, if necessary, repair an old (built in early 1994) Branson Bransonic 8210 MTH Ultrasonic Cleaner. I was unable to find much information on this unit so I decided to trace the circuit board and create a schematic to help with in-circuit testing of the components. My schematic is attached.

I'm curious about what the purpose of the back-to-back diodes are, in series with the drain of the MOSFETs, as well as the diodes between the drain and source. These are D9, D10, D11, D12, D13, D14.

When the top MOSFET is tuned on, current flows to the Drain though the Schottky diode. When a spike is produced when this MOSFET is turned-off, voltage spike currents will flow mostly through the ultrafast D11 diode and bypass the built-in diode of the MOSFET because this path involves two diode voltage drops due to D14 being in series with MOSFET's internal diode and so these diodes improves turn-on time and reduces MOSFET dissipation. Same game with the MOSFET below.

I had already assumed that D11 was to absorb reverse voltage spikes quicker than the MOSFET's internal diode could, but I wondered why you wouldn't just put it directly across the drain and source, leave out D12 and D14, and let both D11 and the internal diode share the load.

I hadn't considered that any current allowed to flow through the MOSFET's internal diode could affect its turn on time. As far as power dissipation goes, the MOSFETs are on a pretty big heat sink, but the power diodes also each have a heat sink of their own, as can be seen in the included board photo.

It's rather MOSFET body diode turn off that is the problem which prompts using these diodes. This is a very common feature in applications where the bridge current may be leading the voltage.

Especially for high voltage and/or old MOSFET types, the body diodes have hideous reverse recovery properties. If the body diode of one mosfet is conducting when the opposite side one turns on, the results may range from anything between high losses, and latch-up of parasitic structures followed by shoot-through and instantaneous destruction. Look for "maximum rate of rise of Vds" or some such parameter

Modern low voltage MOSFETs tend to be quite robust against this phenomenon.

I tested all the components in circuit, as best as I could. Everything seemed OK, so I filled the tank with water and dish soap and tested it using strips of aluminium foil. The foil ended up perforated as expected.

The person who wanted me to test it had actually sold it to someone else who said it didn't work. I guess the person who bought it wasn't using it properly, but then figured it out, because I haven't heard any reports of problems since I returned it.